Epoxy resins possess characteristics such as fine adhesion, corrosion resistance, water resistance, chemical resistance, mechanical strength, dielectric properties, heat resistance, processability, and low contraction rates. Also, their manufacturing cost is low, and thus epoxy resins have been used in various fields. However, the greatest disadvantage of these general-purpose epoxy resins is that they become a high-density, crosslinked polymer after curing, which has high internal stress and results in high brittleness and poor impact resistance. Therefore, the need to improve toughness emerged.
It is known that a liquid rubber can be used as a toughener for an epoxy resin, and liquid carboxyl-terminated polybutadiene-acrylonitrile (CTBN) is the best-known liquid rubber. By introducing a liquid rubber, an epoxy resin may form a rubber microparticle structure of microphase separation after curing. The structure has the effect of absorbing or reducing internal stress, and further improving the toughness, impact resistance, ductility, thermal shock resistance, peel strength, and low-temperature shear performance of the epoxy resin. As a result, the epoxy resin can be used in a wide range of applications. Examples of these applications include being used in adhesives (e.g. adhesives for plastics, metals, or ceramic materials), anti-seismic materials, oil resistant hoses, composite materials (e.g. blades for a wind-power generation, the fuselage of an airplane or yacht, and glass fiber tubes), coatings (e.g. paint for building material, strengthening fluids, insulation coatings, water-proof coatings, and corrosion-resistant coatings), sealants and pouring sealants for electronics, and even in aerospace and military applications.
Although the use of liquid rubber may form a microphase separated structure in an epoxy resin to improve the toughness of the epoxy resin, it usually raises some problems, however. Potential problems are as follows: (1) the decrease of heat resistance resulting from the drastic decrease of glass transition temperature; (2) the significant decrease of elastic modulus and stiffness; (3) losing transparency; and (4) needing a large amount of toughener (generally 10%-20%) to obtain the desired toughness.
Conventionally, different liquid rubbers have been used as tougheners in epoxy resins. For example, a non-reactive liquid carboxyl-terminated polybutadiene-acrylonitrile has been added to epoxy resin to improve toughness, but the non-reactive liquid carboxyl-terminated polybutadiene-acrylonitrile is liable to form incomplete microphase separation and incomplete crosslinking in an epoxy resin, which may lead to insufficient improvement in toughness and poor stability. Moreover, after adding the non-reactive liquid carboxyl-terminated polybutadiene-acrylonitrile, the heat resistance, stiffness, and transparency of the epoxy resin significantly decrease as well.
Liquid carboxyl-terminated polybutadiene-acrylonitrile (CTBN) and liquid carboxyl-terminated polybutadiene (CTPB) have also been used as tougheners in epoxy resins, but the effect of inhibiting the decrease of heat resistance is still not enough. The stiffness and transparency of the epoxy resin cannot be maintained either.
Although liquid hydroxyl-terminated liquid nitrile rubber (HTBN) may exhibit similar properties as liquid carboxyl-terminated polybutadiene-acrylonitrile (CTBN) with a lower cost, HTBN still has some problems, such as needing a large additive amount, and the inevitable decrease in heat resistance, stiffness, and transparency, and so on.
Liquid epoxy terminated polybutadiene-acrylonitrile (ETBN) and liquid epoxy terminated polybutadiene (ETPB), which do not need a pretreatment of prepolymerization, can raise the toughness of the epoxy resin effectively; however, they still cannot solve the problems of decreased heat resistance, stiffness, and transparency. Also, they are not cost-effective.
Adding a small amount of PEG-b-CTBN diblock copolymer and PEG-b-CTBN-b-PEG triblock copolymer may raise the toughness of the epoxy resin effectively. However, diblock copolymer cannot preserve the heat resistance of the epoxy resin, and the glass transition temperature decreases significantly as the added amount increases.
Therefore, with the increasing need for the better performance of epoxy resins manufactures, it has become important to improve the toughness of epoxy resins without negatively affecting other characteristics (such as heat resistance, stiffness, and transparency).